1. Field of the Invention
This invention relates to a pump for injecting gasoline, fuel whose main component is gasoline or low sulfur light oil into a combustion chamber of vehicle engine (hereinafter referred to as fuel injection pump or called fuel direct injection pump or direct injection pump), and more particularly to the same pump using wear resistant ceramics for its piston.
2. Description of the Prior Art
Recently, an improvement of fuel efficiency of an internal combustion engine of a vehicle has been an important problem in viewpoints of environment of the earth. For the reason, studies on realization of so-called in-cylinder direct fuel injection engine in which gasoline or light oil is directly injected into a combustion chamber of the same engine have been made energetically. The fuel injection pump for use in this engine utilizes a reciprocation of a cam to obtain a sufficient pressure for injecting fuel. Receiving this motion, a piston (or called plunger) is slid reciprocatedly along an internal wall of the cylinder so as to inject fuel into the combustion chamber. This sliding condition is very severe. Therefore, by improving the wear resistance and fatigue characteristic so as to reduce a wear loss, the performance and durability of the fuel injection pump can be improved.
Particularly, the gasoline direct injection engine or diesel direct injection engine of a small passenger vehicle uses gasoline having a very low viscosity and a high volatility or low sulfur light oil having a low sulfur content. In the same engines, lubrication of the piston and cylinder is achieved by these fuels. However, because these fuels have a very low lubricity, the insufficient wear resistance and anti-seizure performance of the piston and cylinder may cause a fatal damage of a pump.
As the countermeasure, conventionally, the configuration of the sliding surface has been devised in various ways. For example, stainless base materials excellent in corrosion resistance are used as a material for the piston and cylinder and its surface is finished to be very smooth, and further to improve the wear resistance at the time of sliding, parallel grooves as shown in FIG. 1a or cross hatch lines as shown in FIG. 1b are formed in the sliding surface of the piston in a depth of 2-3 .mu.m max, so as to serve for fuel oil pit thereby increasing sliding lubricity.
According to SAE technical paper 940992, although the shallow cross hatch pattern lines are formed in the cam surface made of metal, the surface roughness of its finished surface is 0.7 .mu.m in terms of Rmax so that the surface is smooth and fine. In this case, there is no directivity in the surface roughness so that uniform finishing is attained. In this case, the surface needs to be finished smoothly and uniformly, and therefore a very troublesome processing is needed. The smooth finishing of the sliding surface may be attained by honing or the like as described in, for example, "Automobile Technical Handbook", vol. 2, page 51 and vol. 4, page 198 by Automobile Technology Association. In case where fuel having a low sliding lubricity is used, particularly, the same treatment needs to be carried out on an internal wall of the cylinder. However, because this kind of the processing, particularly, processing on the internal wall of the cylinder is very troublesome, there is a problem in productivity.
On the other hand, about an aspect of the material, for example, Japanese Patent Application Laid-Open No.61-283759 has disclosed a case in which SIALON or a composite ceramic material of alumina and zirconia (zirconium oxide) is employed for a roller which is directly in a sliding contact with a cam of a direct injection pump of a diesel engine. However, only if the sliding portion is made of ceramic, in case where a mating member which slides correspondingly is made of metal like steel, its surface needs to be finished to be extremely smooth to relax aggressiveness to the mating member and a sliding resistance relative to the mating member. For example, Japanese Patent Application Laid-Open No. 8-232795 has disclosed a case of a diesel engine using low sulfur light oil as fuel in which ceramic is employed for the sliding portion of the direct injection pump. In the same publication, the sliding surface of the same ceramic is finished to 0.5 .mu.m or less in terms of ten-point mean surface roughness Rz or 0.1 .mu.m or less. In the above described cases, because the ceramic sliding surface having a high hardness is finished to be extremely smooth uniformly, usually in a single direction, as described above, it takes much trouble and time for the finishing.
Further, according to Japanese Patent Application Laid-Open No.5-340213, a conventional cheap steel material is used as a sliding portion base material and then smooth film is formed thereon with a hard material, such as CrN, diamond like carbon (DLC) or the like, on the sliding surface of the base material. However, even if these materials are used, the sliding surface needs to be finished to be smooth as described above and additionally, there is a fear that the coated film may be peeled upon sliding. Therefore, there is a possibility that a partial seizure may be caused thereby. As a result, there is a problem that no stabilized wear resistance or anti-seizure performance can be obtained.